1. Field of the Invention
The present invention relates to database systems. More specifically, the invention relates to enabling a group of large object (LOB) columns of a table, or a set of tables, within a database to effectively share a single LOB data segment.
2. Description of the Related Art
The amount of memory space, as for example memory space in a relational database, that is available in computing systems is typically limited, so it is crucial to efficiently allocate the memory space for usage. As such, the determination of where and how to store data within a computing system may be based at least in part upon how much data is to be stored. For example, in some instances, a relatively small amount of data may be stored as inline data in a column of a table, whereas a relatively large amount of data may be stored as an object that is referenced by the table.
A large object (LOB) data type is often used to handle relatively large documents within a database. That is, a LOB data type is generally a data type in a relational database that is used to deal with large objects or documents. The large objects associated with the LOB data type may be in the form of text, graphics, video, or audio. LOB data types generally include binary LOBs (BLOBs) arranged to store binary data and character LOBs (CLOBs) arranged to store character data.
Tables associated with LOBs may generally store large numbers of byte strings, and may be defined as having one or more LOB columns, i.e., one or more columns which are arranged to be associated with LOB data. As shown in FIG. 1, which is a diagrammatic representation of a relational database which includes a table with LOB columns, a table 14 in database 10 includes columns 16. Specifically, columns 16a-c are LOB columns, or columns which are arranged to substantially accommodate rows of LOB data. Each column 16a-c has an associated, dedicated data segment 18a-c within which LOB objects associated with columns 16a-c may be stored. As shown, LOB column 16a is associated with data segment 18a, LOB column 16b is associated with data segment 18b, and LOB column 16c is associated with data segment 18c. 
LOB columns may often store up to approximately four Gigabytes of data by effectively storing indicators in the actual LOB columns that identify LOBs in which data is stored. When a table is created, e.g., using a CREATE TABLE statement, and is expected to contain LOB data types such as a BLOB or a CLOB, a data segment may be allocated for each LOB column associated with the table. The data segment is allocated such that when a LOB is to effectively be stored in a LOB column, the space in the data segment is available for the storage of the LOB. In other words, a data segment is preallocated for each LOB column of a table in anticipation of the storage of LOBs associated with each LOB column. Often, the size of a data segment that is allocated for a LOB column may be fairly significant, as LOBs which may be stored in the data segment may be large. By way of example, if a LOB which is arranged to contain an e-mail message may contain up to approximately 60K bytes, data segments are generally allocated to accommodate LOBs of up to approximately 60K bytes, even though the LOBs may often contain only several hundred bytes.
When a table has a relatively large number of LOB columns, e.g., several hundred LOB columns, the number of data segments allocated for the LOB columns, as well as the size of the data segments, may occupy a substantial amount of space with a database. For example, since the size of string values in an XML document that is stored in a relational database is often unknown, such XML strings may be mapped into LOBs. Within a table, there may be a relatively large number of LOB columns associated with XML data. These LOB columns may also be spread across multiple tables. This implies that a relatively large number of data segments may need to be allocated to accommodate the LOBs into which XML strings are mapped. The allocation of and the maintenance of a relatively large number of data segments may take a significant amount of storage space and time, and, therefore, result in significant performance degradations.
The amount of space which is occupied by a data segment may be specified by LOB storage parameters, e.g., a lob_storage_clause in a CREATE TABLE statement. Because data segments for LOB columns are typically allocated such that LOBs which store a relatively large number of bytes may be accommodated, and the number of data segments may be high, the amount of space reserved for the data segments is often significant. As data is often relatively small, the space allocated for data segments may often be wasted, since the relatively small data may be stored in-line in the columns of the tables, or in the columns of the tables in some instances, instead of in a LOB segment. The space within one data segment that is not used by its associated LOB column may not be utilized, for example, by another data segment or for other purposes within an overall database system. As space within a database is often limited, wasting space which may otherwise be used for other purposes is particularly inefficient.
Therefore, what is needed is a method and an apparatus which enables the space associated with a table, or tables, which contains LOB columns to be efficiently utilized. That is, what is desired is a method and an apparatus which substantially minimizes the amount of space allocated to accommodate LOBs associated with LOB columns of a table in a relational database.